Sacrificial stinger impact absorber

ABSTRACT

A marine pipelaying vessel with a pipeline stinger connected to said vessel, preferably pivotable, and forming a downwardly directed support for pipeline to be laid. The vessel further comprises a stinger suspension assembly between said stinger and said vessel from which said stinger is suspended at least during operation, preferably said stinger suspension assembly allowing to lower and raise said stinger. 
     The stinger suspension assembly includes a sacrificial stinger impact absorber which is adapted to absorb impact energy resulting from a violent stinger motion, e.g. induced by a freak wave. 
     Preferably the impact absorber forms an elongatable structure, e.g. a telescopic structure, which is elongated under impact energy absorption.

The present invention relates to the field of marine pipelaying, whereina pipeline is laid from an offshore vessel onto the seabed.

In this field is it known to use a pipeline supporting stinger tosupport the pipeline from the vessel into the water. The vessel can beof any suitable design, including a monohull vessel, a semi-submersible,a barge, etc. Often said stinger provides a curved or semi-curved pathfor the pipeline, e.g. when the pipeline is assembled or paid out in agenerally horizontal direction from the vessel. The design of thestinger commonly depends on factors like the envisaged water depth,pipeline size and weight, pipeline properties (stiffness, material(s),possible coating, etc), pipeline tension during laying, etc.

For some years pipelaying has been done or at least proposed fordeepwater, which has led to design of very large stingers which allowsupporting the pipeline along a significant arc, up to an essentially 90degrees arc, from horizontal. Stingers have been designed having aweight of more than 100 hundred tonnes, even several hundreds of tonnes.It is noted that the pipeline suspended between the stinger and theseabed also has a very significant weight, which load is carried by saidstinger.

In WO 2005/123556 an example is shown of a pipelaying vessel having astinger and an associated stinger suspension assembly. In said examplethe stinger suspension assembly is connected at one end to the foot of avery large crane on stern of the vessel. Several stinger suspensionassemblies are proposed therein which are adapted to raise and lower thestinger.

In another known design it is known to arrange two cantilevers on thestern of the vessel, which project rearward above the water and eachcarry a winch with a cable from which the stinger is suspended.

Many prior art vessels having a stinger are designed for typical S-lay,wherein the vessel is provided with a pipeline-laying installation withone or more connecting stations, usually welding stations, forconnecting pipe sections in a substantially horizontal orientation on adeck of the vessel. It is noted that stingers are also known on vesselsfor laying reeled pipeline, which pipeline to be laid is stored on oneor more reels.

In the field of pipelaying the continuity of the pipelaying process is aprime factor. The weather and associated wave conditions generally forma limitation on the continuity as pipelaying has to be suspended undercertain unfavourable conditions in order not to compromise the pipelineand/or the pipeline installation system.

The present invention aims to provide an improved pipelaying vessel.

The present invention achieves said aim by providing a vessel accordingto the preamble of claim 1, wherein the stinger suspension assemblyincludes a sacrificial stinger impact absorber which is adapted toabsorb impact energy resulting from a violent stinger motion, e.g.induced by a freak wave. The invention is based on the insight that byproviding one or more specially designed sacrificial stinger impactabsorbers a freak wave incident or the like will potentially only resultin the destruction of the (frangible members of the) absorber while theremainder of the stinger and stinger suspension assembly (as well as anyanchoring points on the vessel and the stinger) are designed to remainintact.

It is known that a freak wave can occur even under relatively calmconditions, so that no precautions can be taken (e.g. changing headingof the vessel, suspending) to counter the effect of the freak wave. Afreak wave can e.g. have a wave height of about 10 metres, with theeffect that the passage of the wave will throw the stinger up and downviolently. Without sacrificial absorber extensive damage must be fearedwhich reduces the operational efficiency of the vessel. The use of asacrificial absorber compared to e.g. a fluidic shock absorber based onfluid pressure or fluid flow (restriction) is advantageous in view ofthe costs for an absorber which has to absorb the enormous impact causedby a violent motion of a stinger.

Preferred embodiments of the vessel and in particular of the impactabsorber are disclosed in the appended claims and in the descriptionwhich follows.

In the drawing:

FIG. 1 a diagrammatically depicts a part of an offshore vessel with astinger, which vessel is suitable, inter alia, for laying a pipeline onthe seabed,

FIG. 1 b in a view from above depicts the arrangement of the two impactabsorbers with the common yoke in the embodiment of FIG. 1 a,

FIG. 2 a in a view from above depicts the yoke of FIG. 1 a with the twoabsorbers,

FIG. 2 b depicts the two absorbers of FIG. 2 a in a front view,

FIG. 3 a depicts in cross-section a impact absorber of FIGS. 1 a-2 b,

FIG. 3 b depict a cross-section over line B-B in FIG. 3 a,

FIG. 3 c depicts a cross-section of the impact absorber of FIG. 3 aalong a different plane, and

FIG. 4, Depicts a cross-section of the impact absorber of FIGS. 1 a-3 cafter impact energy absorption has occurred.

FIG. 1 a shows a stern part of an offshore vessel 1 which is suitable,inter alia, for laying a pipeline on the seabed. The vessel 1 has a hull2 with a working deck 3. The vessel 1 here is provided with apipeline-laying installation of the S-lay type, with one or more weldingstations (not shown) on the working deck 3, for coupling pipelinesections in a substantially horizontal orientation. On the working deck3 there are also what are known as tensioners (not shown) for carryingthe weight of the pipeline which is hanging downwards from the vessel tothe seabed.

Furthermore, the vessel 1 has a stinger 5 which projects outside thehull 2 of the vessel 1 at the rear side of the vessel 1, engages on thehull 2 at an engagement point such that it can pivot about asubstantially horizontal pivot structure 6 and forms a downwardly curvedsupport for pipeline 8 (a part is depicted only) moving towards theseabed.

Furthermore, the vessel 1 has a hoisting crane 20, disposed in thevicinity of the same side of the hull as the stinger 5, which hoistingcrane 20 has a vertical structure fixed to the hull 2. Here, the crane20 is disposed above the location where the pipeline 8 leaves theworking deck 3, on the longitudinal axis of the vessel 1.

The hoisting crane 20 here has a substantially hollow vertical columnwith a foot 22, which in this case is fixed to the hull 2 of the vessel1. The hoisting crane 20 has a jib 24.

The foot 22 of the column here is substantially rectangular, which hasthe advantage that the foot 22 can easily be secured (by welding orusing bolts) to the longitudinal and cross bulkheads of the hull 2 ofthe vessel 1. In a variant which is not shown, the vertical column ispartly or completely a framework of bars.

It can be seen from FIG. 1 that a stinger suspension assembly 40 isarranged between the stinger 5 and the vessel 1 from which the stinger 5is suspended at least during pipelaying operation.

The stinger suspension assembly 40 allows to lower and raise the stinger5 in order to adjust the shape of the support path for the pipelineprovided by the stinger 5.

In this example the suspension assembly 40 includes a winch and cablesystem with one or more cables 41 and with one or more winches 42 thatare arranged on the vessel, here in the vicinity of lower end of thefoot of the crane 20. The cables 41 here extend over cable sheaveassemblies 44 on the upper end of the foot 22 of the crane 20 to acommon yoke 45 or spreader provided with a cable sheave assembly 46.

Between one or more (here two) anchoring points 48 on the stinger 5 andthe common yoke 45 further fixed length cables 49 extend.

In addition, here between the cables 49 and the common yoke 45,sacrificial stinger impact absorbers 50 are arranged.

As will be appreciated operating the winches 42 allows to adjust theeffective length of the stinger suspension assembly 40 and thus toadjust the position of the stinger 5.

As an alternative for the cable system a system including (hydraulic)adjusters could be arranged, a. g. including hydraulic jacks. Again itis proposed to include one or more sacrificial stinger impact absorbers50 between or at the stinger anchoring point and the vessel anchoringpoint of the stinger suspension assembly.

The sacrificial stinger impact absorbers 50 are identical here. They areeach specially designed to absorb impact energy resulting from a violentstinger motion, e.g. induced by a freak wave as explained above. Due tothe enormity of the weight of the stinger each absorber 50 in itself canhave a weight of several tonnes. The absorbers of the design shown herehave a length of about 3 metres and a weight of about 9 tonnes.

The impact absorber 50 has a first connector member 51 and secondconnector member 52 for attaching said impact absorber to said vessel,here to the common yoke 45, and to stinger 5.

In FIG. 2 a temporary links 30 are mounted between the absorbers 50.These links 30 are in practice arranged to avoid sway of the absorbers50 as the vessel is sailing to a location.

The impact absorber 50 forms an elongatable structure, here a telescopicstructure, which is elongated under impact energy absorption. In theexample shown here the absorber 50 lengthens about 1 metre when aviolent stinger motion occurs as will be explained in detail below.

The first connector member 51 includes a tubular housing 51 a and an endcap 53 at one end of said tubular housing 51 a. An eye 55, formed byadjacent plates here, is attached to said end cap 53.

The second connector member 52 includes a piston part 54 slidable ortelescopically received within said tubular housing 51 a. An eye 56,formed by adjacent plates here, is connected to said piston part 54.

As is shown in FIG. 3 a and in the FIG. 3 b the piston part 54, here viaa flange 54 a at the side facing the end cap 53, is fixed to said firstmember 51 (here to the end cap 53 thereof) via one or more frangiblefixations, here a series of break bolts 60 designed to break at apredetermined load on the bolts 60. The threshold for breaking saidfrangible fixations is chosen such that under normal operation saidfixations will remain intact and the impact absorber 50 essentiallyfunctions as a rigid element in the stinger suspension assembly 40.

As can be seen in the figures the piston part 54 has a tapering, hereconical, section 54 a tapering in the direction of elongation of theabsorber 50.

In the tubular housing 51 a a series of sacrificial members is arranged,essentially between the first and second connector members 51, 52 andadapted to plastically deform to absorb impact energy.

The sacrificial members here are embodied as a series of deformableannular members 65 arranged at intervals along the tubular housing 51 a.In total seven of such members 65 are shown in this example.

The annular sacrificial members 65 are embodied here as planar steeldiscs (e.g. having a thickness of several centimetres each) but othershapes can be envisaged as well. Each disc 65 has an opening thereinthrough which said piston part 54 extends, said discs 65 deformingplastically as said piston 54 is moved with respect to the tubularhousing 51 a upon impact absorption.

As can be seen in the figures multiple annular members 65 are arrangedadjacent the conical section 54 a and these members have an openingtherein with a diameter adapted to the diameter of the conical section54 a at said location. Preferably a slight play is present between thediscs 65 and the conical section 54 a so that the break bolt 60 functionis not interfered with.

The discs 65 are mounted removable within the tubular housing 51 a, herethe first connector member 51 includes an annular end cap 57 removableattached to the tubular housing 51 a and having an opening thereinthrough which said piston part 54 a extends. This allows to replace thediscs 65 after having been subjected to plastic deformation. It will beunderstood that the first and second connector members 51, 52 aredesigned to maintain intact upon impact energy absorption by saidabsorber 50. As such the “link” provided by the stinger suspensionassembly between the vessel and the stinger will also remain intact,only the absorber 50 will have an increased length.

In view of the enormous impact to be absorbed and in view of costs it ispreferred that the sacrificial members 65 are of metal, preferablysteel, but other materials, e.g. (reinforced) plastics, can be envisagedas well.

It will be understood that in stead of break bolt other frangiblefixations can be employed.

1. Marine pipelaying vessel with a pipeline stinger connected to saidvessel, preferably pivotable, and forming a downwardly directed supportfor pipeline to be laid, wherein said vessel further comprises a stingersuspension assembly between said stinger and said vessel from which saidstinger is suspended at least during operation, preferably said stingersuspension assembly allowing to lower and raise said stinger, whereinsaid stinger suspension assembly includes a sacrificial stinger impactabsorber which is adapted to absorb impact energy resulting from aviolent stinger motion.
 2. Vessel according to claim 1, wherein saidimpact absorber forms an elongatable structure, which is elongated underimpact energy absorption.
 3. Vessel according to claim 1, wherein saidimpact absorber has a first connector member and second connector memberfor attaching said impact absorber to said vessel and stinger, and oneor more sacrificial members arranged between said first and secondconnector members.
 4. Vessel according to claim 1, wherein said impactabsorber comprises one or more metallic, sacrificial members adapted todeform plastically to absorb said impact energy.
 5. Vessel according toclaim 4, wherein said one or more sacrificial members are designed andarranged so as to be replaced after having been subjected to plasticdeformation and wherein said first and second connector members aredesigned to maintain intact upon impact energy absorption by saidabsorber.
 6. Vessel according to claim 1, wherein said impact absorberhas a first connector member and second connector member for attachingsaid impact absorber to said vessel and stinger, and wherein said firstand second connector members are fixed to each other by one or morefrangible fixations, said frangible fixations being designed to breakwhen subjected to a load surpassing a predetermined break load. 7.Vessel according to claim 1, wherein said impact absorber has a firstconnector member and second connector member for attaching said impactabsorber to said vessel and stinger, and wherein said first and secondconnector members form a telescopic structure, said one or moresacrificial members being arranged between said first and secondconnector members and deforming under elongation of said telescopicstructure.
 8. Vessel according to claim 7, wherein said first connectormember includes a tubular housing and wherein said second connectormember includes a piston part slidable within said tubular housing. 9.Vessel according to claim 8, wherein said first connector memberincludes an end cap at one end of said tubular housing, said pistonbeing fixed to said end cap via one or more frangible fixations. 10.Vessel according to claim 1, wherein said sacrificial members aredeformable annular members, preferably steel discs, having an openingtherein through which said piston extends, said sacrificial membersdeforming plastically as said piston is moved with respect to thetubular housing upon impact absorption.
 11. Vessel according to claim10, wherein said sacrificial members include a series of annular membersarranged at intervals along the tubular housing.
 12. Vessel according toclaim 11, wherein the piston has a tapering, tapering section taperingin the direction of elongation of the absorber, and wherein multipleannular sacrificial members are arranged adjacent said conical part,said sacrificial members preferably each having an opening with adiameter adapted to the diameter of the tapering part at said location.13. Vessel according to claim 10, wherein said one or more annularsacrificial members are mounted removably within said tubular housing.14. Vessel according to claim 1, wherein said stinger suspensionassembly includes a common yoke supporting two absorbers next to eachother, each absorber being connected to said yoke and to an associatedsuspension member, to said stinger.
 15. Vessel according to claim 14,wherein said yoke includes a sheave assembly for a cable hoist arrangedon the vessel as part of the suspension assembly.
 16. Sacrificialstinger impact absorber adapted for a vessel according to claim
 1. 17.Vessel according to claim 2, wherein said impact absorber comprises oneor more metallic, sacrificial members adapted to deform plastically toabsorb said impact energy.
 18. Vessel according to claim 3, wherein saidimpact absorber comprises one or more metallic, sacrificial membersadapted to deform plastically to absorb said impact energy.
 19. Vesselaccording to claim 2, wherein said impact absorber has a first connectormember and second connector member for attaching said impact absorber tosaid vessel and stinger, and wherein said first and second connectormembers are fixed to each other by one or more frangible fixations, saidfrangible fixations being designed to break when subjected to a loadsurpassing a predetermined break load.
 20. Vessel according to claim 3,wherein said impact absorber has a first connector member and secondconnector member for attaching said impact absorber to said vessel andstinger, and wherein said first and second connector members are fixedto each other by one or more frangible fixations, said frangiblefixations being designed to break when subjected to a load surpassing apredetermined break load.